1. Field of the Invention
The present invention relates to laser thermal annealing, and in particular relates to apparatus and methods for performing laser thermal annealing of substrates that do not efficiently absorb the annealing radiation beam at ambient temperatures.
2. Description of the Prior Art
Laser thermal annealing or LTA (also referred to as “laser thermal processing”) is a technique used to quickly raise and lower the temperature of the surface of a substrate to produce a change in properties. An example might include annealing and/or activating dopants in the source, drain or gate regions of transistors used to form integrated devices or circuits. LTA can also be used to form silicide regions in integrated devices or circuits, to lower poly-silicon runner resistances, or to trigger a chemical reaction to either form or remove substances from a substrate (or wafer).
LTA offers the possibility of speeding up the annealing cycle by a factor of 1000 over conventional annealing techniques, thereby virtually eliminating diffusion of dopant impurities during the annealing or activation cycle used on silicon wafers. The result is a more abrupt dopant profile and, in some cases, a higher level of activation. This translates into higher-performance (e.g., faster) integrated circuits.
U.S. patent application Ser. No. 10/287,864 discloses performing LTA of doped silicon substrates using CO2 laser radiation. The laser radiation is focused into a narrow line, which is scanned at constant velocity in a raster pattern across the substrate. However, this approach works well only on relatively heavily doped substrates (i.e., a dopant concentration of about 3×1017 atoms/cm3 or greater), where the absorption length of the laser radiation in the doped silicon is less than or roughly comparable to the thermal diffusion length. Conversely, for lightly doped substrates (i.e., a dopant concentration of about 1×1016 atoms/cm3 or less), the CO2 laser radiation passes through the substrate without imparting appreciable energy to the substrate.
What is needed therefore is way to efficiently perform LTA of lightly doped silicon substrates using radiation that otherwise passes through the substrate without heating, such as CO2 laser radiation having a wavelength of 10.6 μm.